Feasibility Study of Capacitor-Based Smart Washer Sensor Systems for Highway Sign, Luminaire/Light, and Traffic Signal Structures

This paper aims to provide an improvement on the current inspection methods for anchor bolts used in highway sign, luminaire/light, and traffic signal (SLTS) support structures to reduce the risk of structural failure as well as human labor costs. A feasibility study on a capacitance-based smart washer system was conducted. The capacitor-based sensor system design combined with customized polylactic acid three-dimensional printed attachments was proposed and tested. Laboratory tests on multiple sets of sensors were performed through initial testing and tightening–loosening testing; results have shown that the performance of the sensor system follows the working principle of the capacitor and, with a less than 5% error tolerance, a capacitance–pretension relationship was developed in the laboratory. Then the sensor system was installed at a sign structure specimen for long-term evaluation; the 12-month consecutive monitoring results show that the sensor system was able to perform steadily with a proper waterproof cover applied. The change of the capacitance was found at ±5% of the reference value, and torques were applied to verify the measurement results from the sensor system, indicating that the minor capacitance change was caused by a change of temperature. The results have proven the feasibility of this capacitor-based smart washer system that detects the pretension change inside the anchor bolts and performs steadily in a controlled on-site environment with a high durability. In summary, this study provides the possibility of improving the monitoring methods for anchor bolts in highway SLTS structures.